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1. Field of the Invention
The invention relates generally to multi-element composite objects. In particular, the invention relates to multi-element composite objects fabricated from weld-incompatible metals.
2. Description of Related Art
Objects employing combinations of different metals, so-called xe2x80x9ccomposite objectsxe2x80x9d are hardly new. For example, archaeologists have discovered artifacts combining iron and bronze components that date from before the birth of Christ. One basic premise present in these primitive metallurgical developments has carried through the centuriesthe desirable characteristics of different metals may be combined by their use in a single object.
One modern application of this premise is in the aircraft industry. It is well-recognized that every gram of weight that can be removed from an aircraft will pay large dividends by way of reduced fuel consumption, increased performance, or increased payload. Thus, a constant theme in the manufacture of aircraft components is the need to reduce the weight of every component while maintaining or increasing component strength and structural integrity.
One area in which this theme is illustrated abundantly is in the fabrication of gears for use in aircraft. It is typical of such structures that the gear teeth, splines, and bearing races call for materials that have hard surfaces to resist wear, contact fatigue, and bending fatigue. By contrast, gear web and shaft portions are free of such requirements, and are therefore prime candidates for achieving weight reduction.
In order to meet these goals, steel alloy gears with titanium alloy webs and shafts have been proposed. Unfortunately, traditional welding and casting methods are practically ineffective for joining these alloys together. One solution to this problem is described in U.S. Pat. No. 5,492,264 to Wadleigh et al., which is incorporated by reference herein. The ""264 disclosure is directed to a composite object formed by using inertia welding to join first and second dissimilar elements together via a third, mutually-compatible interlayer metal element. The method for forming the composite object involves inertia welding the interlayer to the first element, then inertia welding the second element to the interlayer. In an illustrative embodiment, the composite object is a multi-metal element composite gear, web, and shaft. In a preferred embodiment, a hardened steel alloy gear is inertia welded to an aluminum interlayer, which is in turn inertia welded to a titanium alloy shaft.
While this solution overcomes many of the longstanding problems described herein, it is not without possible shortcomings itself. One area for improvement is in the nature of the interlayer. Despite the fact that aluminum is used extensively in the aircraft industry, there is an impression of vulnerability associated with some aluminum components. This impression is that, while aluminum is a very desirable material for airframes, it is unsuitable for powerplant and transmission applications. Aluminum components introduce a temperature limitation of approximately 350xc2x0 F. above which the metal begins to thermally soften. Although gearboxes do not ordinarily operate at temperatures above 350xc2x0 F. there is a military requirement for gearboxes to function for one hour after all the oil is drained out. The reason for this damage tolerance is to allow the crew sufficient time for escape and egress after the gearbox has been punctured, typically by gunfire or other ordnance. In the civilian world, it is extremely rare for aircraft to be subjected to gunfire, even when such aircraft are operating over high-crime urban areas. However, particularly in the case of helicopters, the same manufacturers make aircraft for civilian and military use. Consequently, the design approach (and many standard components such as gearboxes) are common to both applications.
It can be seen from the foregoing that the need exists for multi-metal composite objects, and methods for their manufacture, that meet weight and strength objectives without sacrificing emergency operating capabilities.
The present invention achieves these and other objects by providing a multi-element composite object composed from first, second, and third metal components, wherein the first metal and the third metal are weld incompatible. The multi-element composite object includes a first component fabricated from a first metal. A second component, fabricated from a second metal, is brazed to the first component. A third component, fabricated from a third metal, is inertia welded to the second component.
The first metal may be provided as a titanium alloy, e.g. a TiNi alloy. The second metal may be provided as steel, e.g., low-carbon alloy or mild steel. The third metal may be provided as alloy steel, e.g., 9310 nickel alloy steel.
In an embodiment,the multi-element composite object is a gear assembly, with the first element of the gear assembly object being a shaft and the second element of the gear assembly being a gear member with hardened teeth surfaces. The first and second components can be mechanically keyed together via an anti-rotational element. The anti-rotational element can be provided as a pin-in-groove arrangement or a twist-fit arrangement.
A method of making a multi-metal composite object including a first component fabricated from a first metal, a second component fabricated from a second metal, and a third component fabricated from a third metal, wherein the first metal and the third metal are weld incompatible, is also disclosed. The first step of the method includes mechanically keying the first component to the second component. Next, the first component is brazed to the second component. Finally, the third component is welded to the second component. Where the first metal is a Ti alloy and the second metal is low-carbon mild or alloy steel, the step of brazing the first component to the second component can include brazing the steel component to the Ti alloy component using a brazing material selected from a group consisting of Ag and Cu. Where the third component is heat-treated steel, the inertia weld joint between the second and third components may be stress-relieved at a temperature sufficiently low so as not to degrade the heat-treated properties of the third component after inertia welding the third component to the second component.
The features of the invention believed to be patentable are set forth with particularity in the appended claims. The invention itself, however, both as to organization and method of operation, together with further objects and advantages thereof, may be best understood by reference to the following description taken in conjunction with the accompanying drawings.